Apatite fission-track (AFT) analyses, applied to Southern Brazil
and Uruguay samples, was employed aiming to understand the low temperature
history of the Dom Feliciano Belt Segment. The Dom Feliciano Belt formed
during the Neoproterozoic to Early Paleozoic, linked to the
Brasiliano/Pan-African Orogeny. Twenty-four samples were dated, and confined
track lengths of twenty samples were measured. The spatial distribution of
ages shows three domains with different evolution cut by shear zones and, or
suture zones in the Dom Feliciano Belt. The Western Domain exhibits AFT ages
> 250 Ma (Permian to Devonian) while the Eastern Domain shows
AFT ages < 230 Ma (Paleogene to Triassic). In the Central Domain,
the AFT ages range from ∼196 to 130 Ma (Jurassic to Early Cretaceous).
The thermal modeling in the domains revealed a complex evolution, with
cooling and reheating phases, and a denudation of ∼2600 m. The AFT ages
clearly postdate the Gondwanide, Paraná-Etendeka and Rio Grande Cone
exhumation history of the Dom Feliciano Belt.

The South America passive continental margin draw itself through several
tectonic episodes that began in the Neoproterozoic (Almeida et al., 2000).
These episodes are related to the breakup of continents, lithospheric and
mantle dynamics. The transitions between Uruguay and Rio Grande do Sul State,
Brazil, represents an evolutionary key area for the Gondwana Breakup of the
lithospheric plates of South America and Africa. Several tectonic events took
place during the Neoproterozoic to Cambrian, including accretionary orogenic
processes that marked the transition from stable to unstable tectonic
conditions. These processes, such as granite magmatism and metamorphism,
(e.g. Piedra Alta and Nico Perez Terrane, in Uruguay, and Sul-rio-grandense
Shield basement, in Brazil) have been recorded in many places (Bossi et al.,
1998; Preciozzi et al., 1999; Chemale Jr. et al., 2012). Post-collisional
processes occurred in the passive margin, mainly including foreland
tectonics, extension-related intrusions, and isostatic adjustments (Chemale
Jr. et al., 2005, 2012; Basei et al., 2008; Rapela et al., 2011; Saalmann et al.,
2011). The Gondwana's formation occurred during
Neoproterozoic to Cambrian by fusion of at least five cratons, among them the
Rio de la Plata, Kalahari and Congo Cratons. The interactions among these
cratons formed the Dom Feliciano Belt in Brazil and Uruguay (Philipp et al.,
2016). In the Late Paleozoic, the Gondwanide Orogeny climate changes,
including glaciations records in the Itararé Formation, Brazil, and
tectonic affected the Gondwana margins. The cold climate gradually changes to
temperate and humid atmosphere. The cold climate gradually changed to the
temperate and humid atmosphere. A sedimentary sequence of siliciclastic
rocks, associated with intrusion and extrusion of the Paraná-Edendeka
Suit (Zalán et al., 1990), then overlain The Precambrian basement.

The tectonic evolution of the Uruguay and Rio Grande do Sul State in Brazil,
is marked by metamorphic rocks and intrusions reworked of the Neoproterozoic,
partly overlain by a thick sequence of Paraná rocks (Zalán et al.,
1990). Milani et al. (1997) characterized six megasequences for the Paleozoic
and Mesozoic intervals of the Parana Basin, where the depositional history
was controlled by tectonism and climatic changes. Subparallel fault and
fracture zones to the coastline were reactivated and over them the deposition
of the Pelotas Basin (Fig. 1; Tomazelli and Willwock, 2000). Many of these
records are known in Southern Brazil through thermochronological studies
(Gallagher et al., 1994, 1995; Borba et al., 2002; Tello et al., 2003;
Hackspacher et al., 2004; Gomes, 2011; Hiruma et al., 2010;
Franco-Magalhães et al., 2010; Siqueira-Ribeiro et al., 2011; Chemale Jr.
et al., 2012; Karl et al., 2013; de Oliveira et al., 2016). These works
suggest reactivation of the Precambrian structures and plate adjustment of
breakup associated with the South Atlantic rift evolution. In addition to
studies in restricted areas that suggest denudation in the NW cratonic
interior of the Sul-rio-grandense Shield.

Figure 1Simplified geological map of the Dom Feliciasno Belt in Uruguay and
Rio Grande do Sul State, Brazil, and shear zones showing samples locations.
Regional distribution of AFT ages (±1σ) of the working area. In
the blue boxes, AFT ages of West Domain of Dom Feliciano Belt; in the white
boxes, AFT ages of Central Domain; and in the yellow boxes, AFT ages of East
Domain (adapted after Castillo Lopes, 2009; Chemale Jr. et al., 2012; de
Oliveira et al., 2016; and Gruber et al., 2016).

In order to improve the understanding of the Gondwana breakup at the Southern
South America by thermochronological studies twenty-four new apatite
fission-track data of granitic and gneiss rocks from the Uruguay and Southern
Brazil were obtained. The results of this study provide a simplified model of
the large-scale geodynamic evolution of the Dom Feliciano Belt Segment
through time should be given, especially uplift and erosion (cooling),
subsidence and magmatism (heating), and timing of the movement of large
onshore faults and shear zones. It is a significant effect on the
evolutionary history of the Sul-rio-grandense and Uruguay Shields.

The Sul-rio-grandense and Uruguay Shields, situated in the Southwest
Gondwana, originated from divergent movements between the two lithospheric
plates of South America and Africa, with subsequent opening of the South
Atlantic. These shields comprise rocks generated between Archean and
Ordovician including Dom Feliciano Belt.

The Dom Feliciano Belt forms a discontinuous belt, bounded by NE–SW and
NW–SE oriented regional Brasiliano Shear Zone (Fig. 1). This Belt is
subdivided into three units by Fernandes et al. (1995), which are denominated
Central, East, and West Dominions, from gravimetric data and separated by
suture and shear zones. The Western Domain separated from the Central by
NE–SW trending Caçapava Suture (Fig. 1), being formed by
granitic-gneissic-magmatic rocks associated with multiple magmatic
intrusions, which are predominantly located along the Major Gercino Shear
Zone (Fig. 1; Chemale Jr. et al., 2012). The Major Gercino Shear Zone has a
NE–SW oriented and is an important formation that separates the magmatic arc
granites to the East and a folded supracrustal belt to the West. This domain
is formed by Taquarembó and São Gabriel Terranes, Brazil (Fig. 1).
The Taquarembó Terrane is composed of Paleoproterozoic granulites with
protolith ages of 2.45 Ga (Hartmann et al., 2000), several granite
intrusions with age at 0.65–0.55 Ga (Gastal et al., 2006) and
Neoproterozoic to Eopaleozoic volcano-sedimentary rocks. The São Gabriel
Terrane consists of ophiolite slabs, volcanic-arc-related rocks,
granite-gneiss rocks with a Neoproterozoic juvenile signature,
volcano-sedimentary rocks and late- to post-orogenic granite intrusions and
volcano-sedimentary rocks of Camaquã Basin (Fig. 1; Chemale Jr., 2000;
Hartmann et al., 2000; Saalmann et al., 2011).

The Central Domain is formed by part of the Camaquã Basin, Tijucas
Terrane and a small part of the Pelotas Batholith (Fig. 1). The Tijucas
Terrane (Fig. 1) is composed of metasedimentary rocks intercalated with
metavolcanic rocks of the Porongos Complex with ages from ca. 0.5 to 2.8 Ga
(Chemale Jr., 2000; Saalmann et al., 2011; Basei et al., 2008; Gruber et al.,
2016), granitic-gneissic rocks of the Encantadas Complex. The Encantadas
Complex is interpreted as a Paleoproterozoic active continental arc (Saalmann
et al., 2011; Philipp et al., 2008) and granitic intrusions and sedimentary rocks. The Pelotas
Batholith bounded to the West by the Canguçu Shear Zone, is represented
by granite rocks with ages of 650–550 Ma and interpreted as a continental
magmatic arc (Hartmann et al., 2000; Philipp and Machado, 2005), tectonically
reworked in the Paleoproterozoic (Fernandes et al., 1995). The Camaquã
Basin was generated in the late- to post-orogenic sedimentary cycles of the
Brasiliano Orogeny in the Dom Feliciano Belt (Chemale Jr., 2000). The basin
is NE–SW-elongated and is composed of the following four units:

Guaritas Group dated at 547±6.3 Ma and
473.7±9.4 Ma (Almeida et al., 2012). The Eastern Domain separated from
the Central by Canguçu-Major Gerciano Shear Zone (Fig. 1). This domain is
formed by Pelotas Batholith, previously described.

In Uruguay (Fig. 1), the Dom Feliciano Belt separates into three different
units by the Sierra Ballena Shear Zone:

Nico Pérez Terrane

Piedra Alta
Terrane; and;

Cuchila Dionísio-Pelotas Batholith (Fig. 1).

The Sierra Ballena Shear Zone is part of a transcurrent system that divided
the Dom Feliciano Belt into two different domains: Schist Belt and Granitic
Belt (Bossi and Campal, 1992; Bossi et al., 1998; Teixeira et al., 1999). The
Schist Belt is correlated in Brazil to the Porongos Group (Basei et al.,
2008) and the Granite Belt is correlated with the Pelotas Batholith
(Oyhantçabal et al., 2010; Philipp and Machado, 2005).

The Piedra Alta Terrane defined by Bossi et al. (1998), located in the most
Western part of Uruguay – West of the Sarandi del Yí Shear Zone,
consists of three Paleoproterozoic belts, with E–W orientation and formed by
metavolcanic-sedimentary rocks intercalated by a granite-gneissic
chrono-correlated (Dalla Salda et al., 1988; Cingolani et al., 1997; Hartmann
et al., 2000). The Piedra Alta Terrane is the root of a juvenile
Paleoproterozoic magmatic arc (2.2–2.0 Ga, Hartmann et al., 2002).
According to Bossi et al. (1998), Campal and Schipilov (1999) and Teixeira et
al. (1999) this terrane was tectonic stable at ca. 1.7 Ga. Mesoproterozoic
ages were reported by Bossi et al. (1998) for the Sarandí del Yí
Shear Zone suggesting that the Rio de la Plata Craton has been tectonically
reworked.

The Cuchilla Dionísio-Aiguá Batholith (Gaucher et al., 2009),
located to the East of the belt (Fig. 1), is characterized by Neoproterozoic
to Paleoproterozoic granitoids and metagranitoids rocks (Bossi et al., 1998;
Preciozzi et al., 1999) and limited from Nico Pérez Terrane by the Sierra
Ballena Shear Zone (580–550 Ma, Ar-Ar, Oyhantçabal et al., 2010). This
shear zone has continuity in Southern Brazil by the Canguçú Shear
Zone (Fernandes and Koester, 1999), which was active at the end of the
Neoproterozoic. The Sierra Ballena Shear Zone is marked by strong linear
negative gravity anomalies (Hallinan et al., 1993) and probably controlled
the intrusion of calc-alkaline granites (Bitencourt and Nardi, 1983). The
Aiguá Batholith (Fig. 1) has been correlated with the Pelotas Batholith
and is interpreted as the root of a Neoproterozoic magmatic arc.

Southern South America is affected by a very extensive glaciation at the
Carboniferous-Permian (350–250 Ma) (Mcloughln, 2001) with the register of
the Itararé Group (de Freitas-Brazil, 2004). At the same time, the
Gondwanide Orogeny compressed and deformed the cratonic areas on the West and
South Gondwana margins (e.g. Sierra de la Ventana, Uruguay) (Ramos, 1988) and
formed the backbone of Uruguay and Rio Grande do Sul basement (Milani, 1997).
In addition, the absence of records of sedimentation in Southern Brazil
confirms that the basement was active (Milani, 1997). While in the
Permian-Triassic the development of the lineaments with E–W direction
parallels the zones of oceanic fractures, which can be related to the later
development of the South Atlantic (Zalán et al., 1990). Parts of Gondwana
rotated over the Southern during the Late Paleozoic (Mcloughln, 2001). The
pre-existing structures (from Pre-Cambrian, Paleozoic and Triassic)
collaborated with the South Atlantic opening and accommodated the deposition
of Paraná Basin (Milani, 2000). The Paraná Basin has an elongated
shape of almost 1 100 000 km2 and extends in a Southwest-Northeast
direction throughout Brazilian Territory, Paraguay, Uruguay, and Argentina.
According to Milani (1997) and Zerfass et al. (2003) this tectonic pattern is
observed on the Gondwana Supersequence II (Rio Bonito, Palermo, Irati, Serra
Alta, Rio do Rastro and Pirambóia formations). Franco-Magalhães et
al. (2010) suggest that up to ∼6000 m of siliciclastic sedimentary
rocks fill the old erosional topography with the Paraná flood basalt
surface.

In the Neocomian (145–130 Ma) the rift phase began, which was followed by
ascending intrusive bodies through fractures of the crust. Among them the
Serra Geral magmatism with an age 134.5 Ma (Pinto et al., 2011; Janasi et
al., 2011), which used to ascend by older structures with NW–SE direction.
After the Serra Geral magmatism the substrate of the region was shaped
(Milani, 2000) and on subparallel faults to the coastline of the basement was
deposited the Pelotas Basin (Tomazelli and Villwock, 2000). The Pelotas Basin
is located in South Atlantic between Brazil and Uruguay. It is limited in its
Northern part with Santos Basin through the Florianopolis Fracture Zone. In
the Southern, it is limited by Chui Lineament, which separated it from Punta
del Este Basin, Northern Uruguay. Structural data of these margins indicate
that Pelotas Basin was controlled by tectonic stages of at South Atlantic
pre-syn and post-rifting and affected by high-angle extensional faults
(Bassetto et al., 2000; Contreras et al., 2011). Pelotas Basin covers more
than 200 000 km and includes Rio Grande Cone (Fig. 1). The Rio Grande Cone
is a depositional feature formed during the Neogene and ultra-deep, which can
reach 3600 m (Fontana, 1989).

The Late Cretaceous stable conditions have been recorded from fission-track
ages (90–60 Ma, Gallagher et al., 1994), which refer to the sedimentation
cycle in SE South America. Gallagher et al. (1994) suggest these
thermochronological ages represent the Paraná-Etendeka volcanic suite
event. These authors proposed an apatite fission-track (AFT) age-elevation
relationship coupled with the occurrence of older ages (with AFT ages
> 300 Ma) within very close geographic proximity to younger ones
(90–60 Ma). The exhumation and denudation of high-elevated areas during the
Late Cretaceous, in response to the drift of South America over of the
Trindade hotspot, were dated by thermochronological data and the AFT ages was
close to 90 Ma (Meisling et al., 2001; Tello et al., 2003; Hackspacher et
al., 2004).

Data and information from tectonic events associated with residual weathering
and sediments over paleosurfaces from Southeastern South America are compared
to potential correlative sequences either in a continental interior as Santos
Basin and to regional rifting basins. Uplift and ages are interpreted from
AFT in literature. In the Pelotas Basin, the evidence of exhumation of the
coastal mountain source area in the Paleogene is a thick sequence of
sediments prograding outward, being known as Rio Grande Cone (Castillo Lopes,
2009). Franco-Magalhães et al. (2010) suggest that Paleocene-Eocene
thermochronological ages indicate block faulting and exhumation of the flanks
along of the Paleogene rifts. In the onshore, from Paleocene to Miocene
(60–20 Ma), half-grabens trending NNE–SSW and are filled up by terrigenous
sediments of more than 800 m (Melo et al., 1985; Almeida et al., 2000;
Cobbold et al., 2001; Meisling et al., 2001; Salamuni et al., 2003; Karl et
al., 2013).

The South Atlantic opening still involves numerous uncertainties, like
locations and magnitudes of overlaps between continental rocks, but it is
accepted that initiated in the Southern Argentinean–African margins and
propagated Northward to the Brazilian and African margins (Moulin et al.,
2010).

Twenty-four new samples were collected along the Western, Central and Eastern
Domains of the Dom Feliciano Belt. We focused on the area where we considered
there to be the best opportunities to identify the evolution of the Gondwana
breakup. In the Western Domain, three samples were collected in the Piedra
Alta Terrane, and three samples in the Nico Perez Terrane (Uruguay). In
Southern Brazil, in this domain, two samples were collected in the
Taquarembó Terrane and one sample to the Western of Caçapava Suture
in the São Gabriel Terrane. In the Central Domain, one sample was
collected to the East of Caçapava Suture in the Camaquã Basin, and
one sample in the Tijucas Terrane and two samples to the West of Sierra
Ballena Shear Zone in the Cuchilla Dionísio Terrane (Uruguay). In the
Eastern Domain, ten samples were collected in the Pelotas Batholith (Brazil),
one sample in the Aiguá Batholith and two samples in the Cuchilla
Dionísio Terrane (Uruguay). In total, nine samples from Uruguay and
16 samples from Southern Brazil (Fig. 1; Table 1).

Apatite minerals were separated and isolated from each sample following
procedures outlined in Dumitru (1999) using standard gravimetric and magnetic
mineral separation techniques. The natural apatite was mounted in epoxy,
polished and etched in 5N HNO3 for 25 s at 25 ∘C to
reveal the spontaneous fission-track. All AFT ages were performed using low-U
mica sheets as external detector method (Gleadow and Lovering, 1977). Apatite
samples were irradiated at the IPEN-CNEN nuclear reactor, São Paulo
State, Brazil, with two glass neutron dosimeters (CN5 and CN1) with known
uranium content together with Fish Canyon Tuff age standards and Durango
apatite age standards (Hurford and Green, 1983; Green 1985). After
irradiation, mica detectors were etched in 48 % HF for 18 min at
20 ∘C to reveal the induced fission-tracks. The AFT analyses on all
samples were performed at the Federal University of Rio Grande do Sul,
Brazil, using a Zeiss Axioplan 2 imaging with
Autoscan® system by the first author. AFT
central ages were calculated according to the ξ-calibration method
(Hurford, 1990; Hurford and Green, 1983). The AFT age errors are quoted at
the 1σ confidence level and were derived by the conventional method
(Green, 1985). The χ2-squared test was used to quantify age homogeneity;
when P(χ2) > 5 %, fission-track samples contain a
single age population (Gallagher et al., 1995). The AFT age calculation,
their dispersion, and distribution were obtained by the
Trackkey® v. 4.2 Software (Dunkl, 2002).

Confined track lengths were measured on horizontal confined fission tracks
following procedures Laslett et al. (1982) to construct an apatite
fission-track length-frequency distribution. The apatite composition in term
of chlorine and fluorine was carried out in the Department of Geology,
Federal University of Rio de Janeiro State, Brazil, using a JEOL JXA-8230
Electron Microprobe with 15 kV accelerating voltage, 25 nA current, and
5–20 µm beam diameter.

The thermal histories were modeled using AFTSolve 1.4.1 (Ketcham et al.,
2000). The time–temperature evolutions tested against the
thermochronological data set are determined by using the published geological
history of the area. The inversions modeling was run with 10 000 randomly
chosen time–temperature (t--T) histories for each sample. Thus, the model
paths were available to clearly differentiate between “goodness of fit” and
“acceptable fit” solutions depending on the ages and lengths parameters and
the geological background and cooling history of different areas. The initial
inverse modeling is based on the best forward model revealed. We restricted
the conditions for inversion modeling as follows:

a.

as initial constraint the end of the Brasiliano Orogeny (ca.
550–450 Ma);

b.

a large t−T box was imposed with T closed limits in the base
of the apatite partial annealing zone temperature (120 ∘C) and at
the paleosurface temperature (20±10∘C);

c.

a present mean
surface temperature of ca. 20±5∘C provided the final modeling
constraint. The annealing model used was of Ketcham et al. (1999), with Cl
content values as a kinetic parameter and Dpar (ppm) values
(Donelick et al., 2005).

4.1 Apatite fission-track age distribution

The AFT ages and confined track lengths measured from studied samples are
presented in Table 1, Figs. 1 and 2. The thermal histories for all samples
are presented in Figs. 3–5. AFT central ages measured in the West
Domain range from 383.4±40.9 (sample RS-6) to 250±21 Ma (sample
RS-12). In the Central Domain the AFT central ages range from
196.2±14.7 Ma (sample RS-11) to 130.2±13.3 Ma (sample RS-7), and in
the East Domain the AFT central ages range from 262.2±29.9 Ma (sample
PJV-5) to 38±2.5 Ma (sample U-1), which closely matches the range
determined by de Oliveira et al. (2016) and Kollenz (2015) (Fig. 2). The AFT
age distribution allows the separation of the Western Domain with older ages
(> 250 Ma) and the Eastern Domain with younger ages
(< 230 Ma). One sample, taken from Pelotas Batholith revealed a
central age of 262.2±29.9 Ma (sample PJV-5) much larger than the AFT
ages from this domain. Zalán et al. (1990) suggest the development of the
lineaments with E–W direction related to the South Atlantic rifting and the
opening of the ocean in the Permian-Triassic. On the other hand, one sample
from the Aiguá Batholith (Uruguay) revealed a central age of
38±2.5 Ma (sample U-1) much lower than the AFT ages from this domain.
Franco-Magalhães et al. (2010) suggest that Paleocene-Eocene AFT ages
indicate normal re-activation of old Brasiliano/Pan African Shear Zones along
the Paleogene rifts. Thus, the Eastern Domain without these two samples, in
particular, has AFT ages ranging from 226.7±28 Ma (sample U-8) to
70.3±5.2 Ma (PJV-7).

Figure 2Variation of fission-track ages (Ma) and elevation (m) for the samples from
Uruguay and Southern Brazil. Data from de Oliveira et al. (2016) and
Kollenz (2015) to comparation.

Figure 3Thermal history of samples from the Western Domain. The numerical
modelling used the software code AFTSolve (Ketcham et al., 1999). The
temperature and time paths and the confined track length distributions are
overlain by a calculated probability density function (best fit). The diagram
on the left show four different fits: green paths are the acceptable fit;
pink paths are the goodness of fits; and blue-red line is the best fit. The
red line represents the moment of the heating. G.O.F.: goodness of fit, N:
number of single grain ages and measured track length (MTL).

4.2 Confined track-lengths

The dataset has more than 20 grains per sample (except for the samples U-7
and U-10 from Uruguay and, PJV-10 and PJV-14 from Brazil). The confined track
lengths measured are unimodal with a range from 14 (sample U-3) to 106
(sample PJV-17), suggesting, in general, thermal histories with slow-cooling.
In addition, they do not display the boomerang form, which is characteristic
of a rapid cooling (Gallagher et al., 1994; Green, 1985). The Measured Track
Length (MTL) ranges from 9.23 µm (sample PJV-7) to
12.53 µm (sample U-4) with an average of 10.8 µm. In the
West Domain the MTL ranges from 10.02 µm (sample U-3) to
12.53 µm (sample U-4), and in the East Domain, the MTL ranges from
9.23 µm (sample PJV-7) to 12.02 µm (sample RS-8).
According to Gallagher et al. (1994), the mean confined track length
decreases slightly with age. The relationship between AFT age and the shape
of the confined track length distributions is characteristic of samples that
recorded various degrees of thermal annealing which occurred prior to a
common episode of cooling (Gallagher et al., 1994).

Only one sample of the West, Central and East Domains did not present
confined track length. The sample of the West Domain is located in the Piedra
Alta Terrane, Uruguay, (U-10) with an AFT age of 121.9±19 (Fig. 1,
Table 1). The sample of the Central Domain is located in the Tijucas Terrane,
Brazil, (RS-7), whose AFT age is 130.2±13.3 Ma. The sample of the East
Domain is located in the Pelotas Batholith, Brazil, (PJV-5), which shows AFT
age of 262.2±29.9 Ma. These samples could not be modeled due to lack of
confined track length, suggesting that apatites undergo high temperature
(> 120 ∘C). The fission-track will be annealed, and the
fission-track system will recount. In the Early Permian, the Paraná Basin
expanded considerably reaching Rio Grande do Sul State, Brazil, with
structural modification in the substratum of the basin by subsidence and
sedimentation.

The apatite chemical compositions were determined in order to establish the
influence of fluorine and chlorine content on the annealing process. The
analyzed grains are all fluorapatite (Donelick, 1991) with mean chlorine
content range from 0.003 to 0.259 wt % and low resistance to annealing
(O'Sullivan and Parrish, 1995).

4.3 Age-elevation

The correlation between AFT ages and elevation study here show old ages often
occur within very close geographic proximity to young ones. All AFT central
ages are much younger than the Brasiliano/Pan-African Orogeny that affected
the Sul-rio-grandense Shield and Uruguay rocks before 500 Ma. The collective
AFT dataset from Uruguay shows a systematic correlation between AFT age and
elevation (Fig. 2). This relationship indicates a positive correlation
between elevation and distance West and East of the Sierra Ballena Shear
Zone, with an apparent uplift rate calculated of 40 m Ma−1 during the
Eocene. We, therefore, interpret this relationship as the reflection of the
regional topography which is characterized by an elevated interior plateau
separated from the coastal region by a Sierra Ballena Shear Zone. Based on
geomorphology, the Sierra Ballena Shear Zone has separated into three
sectors: Southern, Central and Northern. The Northern and Southern sectors
are a strong contrast between the quartz mylonites and mylonitic porphyries
of the shear zone (Schist Belt), which are more resistant to weathering, and
the country rocks. In the central sector, the Granite Belt (Aiguá
Batholith) and the low relief are associated with Mesozoic Basin
(Oyhantçabal et al., 2010). This implies that the Uruguay region probably
attained positive relief during the Late Cretaceous. During the Paleocene,
the area was eroded and exhumed, which probably contributed to fill basins
formed by rifting (Oyhantçabal et al., 2010). However, only the U-1
sample has an AFT age of 38±2.5 Ma indicating that it was cooling during
uplift and erosion in the Eocene, at about the time that Rio Grande Cone was
formed in the Pelotas Basin (Castillo Lopes, 2009).

In Southern Brazil, the correlation between AFT age and elevation (Fig. 2) is
not obviously related to age and distance to the coast, through the ages show
a tendency to increase with altitude. The majority of the samples were
collected from elevations < 353 m; although only one sample from
somewhat higher elevation (422 m) was collected from the summit surfaces of
the Canguçu Shear Zone (Table 1, Fig. 2).

4.4 The thermal modeling results

The thermochronological data exhibit a complex thermal history at the Uruguay
and Brazilian margins, which had record in the Permo-Carboniferous and
Gondwanide Orogeny (Figs. 3, 4 and 5). For example, the Uruguay registers
heating processes in the Anisian-Tithonian to Aptian, possibly associated
with the Jurassic-Cretaceous continental rifting that affected the region.
Already in Southern Brazil the thermochronological data show the Paraná
Basin deposition, the Serra Geral magmatism, and the Rio Grande Cone
evolution. The thermal modeling shows that the presently exposed rocks of
Uruguay and Southern Brazil were at temperatures around 70 ∘C during
the Jurassic-Cretaceous period. In this way, the rocks present at the surface
were at depths of at least ∼2360 m at that time, for a paleogeothermal
gradient of 25 ∘C km−1 (Raab et al., 2002; Luft et al., 2005;
Tinker et al., 2008). It could be assumed that the thermal modeling results
reflect ongoing slow exhumation during the Jurassic-Cretaceous. In addition,
our confined track length data, presenting a wide distribution and short mean
values (Table 1) could be a result of continuous slow cooling from
temperatures in excess of 120 ∘C to surface temperatures (Gleadow,
1986).

The Western Domain of Dom Feliciano Belt (Fig. 3) is characterized by an
initial very fast period of cooling during the Cambrian from 120 to
60 ∘C in the Taquarembó Terrane, Brazil (sample RS-5). During
the Devonian from 200 to 70 ∘C (sample U-2) and 90 to 70 ∘C
(sample U-3) in the Nico Perez Terrane, Uruguay. This was followed by a slow
period of cooling to the Carboniferous-Permian from 70 to 60 ∘C
(Fig. 3). After a final, a very fast period of cooling in the Eocene to
Oligocene to almost surface temperatures (∼25∘C). In the
Devonian, the Piedra Alta Terrane is characterized by a rapid period of
reheating from 30 to 80 ∘C (sample U-4), which extends until the
Late Jurassic as a slow period of reheating from 20 to 80 ∘C (sample
U-5).

The Central Domain of Dom Feliciano Belt (Fig. 4) displays slow and fast
intercalated periods of cooling since the Devonian from 80 to 50 ∘C
in the Camaquã Basin (sample RS-11). However, the Cuchilla Dionísio
Terrane, Uruguay (sample U-7), reveals a very fast period of cooling at the
Triassic boundary from 80 to 50 ∘C, followed by a period of
reheating in the Jurassic to 50 to 80 ∘C (Fig. 4). After a final
slow period of cooling in the Cretaceous-Paleogene to 80 to 60 ∘C.
In the Eocene to Oligocene both Camaquã Basin, Brazil, and Cuchilla
Dionísio Terrane, Uruguay, are characterized by a fast period of cooling
to almost surface temperatures (∼25∘C).

The East Domain of Dom Feliciano Belt is characterized by periods of
variation of temperature mainly in the Pelotas Batholith. During the
Ordovician-Late Carboniferous, the region presents initial periods of
reheating from 110 to 170 ∘C (samples PJV-2, PJV-4 and PJV-17,
Fig. 5), at the same time a fast-moderate cooling from 170 to 30 ∘C
(sample PJV-6) occurs. In the Jurassic-Cretaceous a slow cooling is observed
(∼60∘C) in both the Pelotas Batholith, Brazil, and in Cuchilla
Dionísio Terrane, Uruguay. However, the Pelotas Batholith reveals a very
fast period of heating in the Triassic (sample PJV-6) that extends to the
Oligocene (sample PJV-4).

The exposes rocks from the uplifted region at the South America passive
continental margin in Southern Brazil and Uruguay are formed at the
Paleoproterozoic due to the collision of Rio de la Plata, Kalahari and Congo
cratons reworking. There were subsidence and exhumation which marked the
tectonic evolution in Southern Brazil and Uruguay (Almeida et al., 2000;
Gomes, 2011; Rapela et al., 2011; Saalmann et al., 2011; Chemale Jr. et al.,
2012; Philipp et al., 2016). In this crustal domain, during the Gondwana
Orogeny, important NNE–S-trending lineaments were generated (e.g. Sierra
Ballena-Major Gercino, Sarandí del Yí in Uruguay and, Canguçu
and Ibaré in Brazil) and the oblique movement of the collision tectonic
blocks formed the main suture zones (Caçapava, Porto Alegre and São
Gabriel in Brazil). These movements are related to both a tangential tectonic
regime and a transcurrent, which are characterized by continent-continent
collision and low-angle planar structures (Fernandes et al., 1995).

5.1 Ordovician to Permian (Gondwanide Orogeny)

The first episode of cooling was recorded in the samples of the Piedra Alta
and Nico Perez Terranes in Uruguay and, Taquarembó and São Gabriel
Terranes in Southern Brazil from ∼500 Ma extending up to 260 Ma
(Fig. 3). This age interval coincides with three known events in the region
of the Western Domain of the Dom Feliciano Belt, being:

a.

the time of cooling through the 300–350 ∘C internal from the
biotites and muscovites whose K-Ar age is ca. 570 Ma from mylonites of Major
Gercino Shear Zone in Uruguay (McDougall and Harrison, 1999). This age
indicates later brittle reactivation of the shear zones in Devonian;

b.

the reactivation of faults related to a late thermal-tectonic event, which
have an age range of 540 to 530 Ma (mica Ar-Ar and biotite K-Ar), and were
probably responsible for the formation of the Camaquã Basin (Philipp and
Machado, 2005);

c.

the shear zone reactivations can be associated with the stabilization
phase with low exhumation events of tectonic blocks correlated to the
Paraná Basin evolution (Hackspacher et al., 2004). Enhanced cooling
around 500–260 Ma, as indicated by the model (Fig. 3), could explain a
denudation rate around 40–560 m in Southern Brazil (sample RS-6) and
Uruguay (sample U-2), respectively. At the same time the temperature
decreased from 200 to 60 ∘C, corresponding to a greater extensive
glaciation (Mcloughln, 2001).

Moreover, we interpret the age of 262.2±29.9 Ma (sample PJV-5) in the
East Domain as the reflex of the development of lineaments with E–W
direction related to the South Atlantic rifting and the opening of the ocean
in the Permian-Triassic.

5.2 Triassic to Jurassic (Post Gondwanide Orogeny)

The AFT ages in the Central and East Domains imply a phase of cooling between
250–150 Ma (Figs. 4 and 5). The samples coming from regions as Camaquã
Basin, Tijucas Terrane and Pelotas Batholith in Brazil, and Cuchilla
Dionísio Terrane in Uruguay close to tectonic lines (Sierra Ballena,
Canguçu, Major Gercino, Passo do Cação Shear Zones and,
Caçapava and Porto Alegre Sutures), which are interpreted as reactivated
zones (Philipp and Machado, 2005; Oyhantçabal et al., 2010; Gomes, 2011;
de Oliveira et al., 2016). Some authors suggest that shear zone
reactivations, in the Triassic (206 and 230 Ma), observed in mylonites at
Northern Major Gercino Shear Zone are associated with a thermal pulse
connected to an early phase of the opening of the South Atlantic (Passarelli
et al., 2010). The Major Gercino Shear Zone is a crustal discontinuity that
encompasses several anastomosed shear zones, striking NNE and NE with
dominant transcurrent kinematics and in which syn-tectonic calc-alkaline,
peraluminous and alkaline granites occurred (Oyhantçabal et al., 2010).
The Triassic is associated with the formation of a NW-trending grabens, which
are probably related to a relaxation of the compressional stress field.

The episodes during the Late Triassic and the Early Jurassic (∼200 Ma)
would be related to the distensive regime that operated along the Gondwana
margin in the Paleozoic (Keeley and Light, 1993; Milani, 2000; Zerfass et
al., 2003), which formed Graben-type basins in Southern South America. In
this case this structure is probably associated with thermal pulse to an
early phase of the opening of the South Atlantic Ocean (Melo et al., 1985;
Riccomini et al., 1989; Almeida and Carneiro, 1998; Almeida et al., 2000;
Cobbold et al., 2001; Meisling et al., 2001; Salamuni et al., 2003; Tello et
al., 2003). Reheating events during Late Triassic and Early Jurassic
(∼200 Ma) is denoted by a temperature increase of
∼60–80 ∘C in the Nico Perez Terrane (Fig. 4, sample U-7) and
50–70 ∘C in the Pelotas Batholith (Fig. 5, sample PJV-6).

5.3 Cretaceous to Recent (Syn- and Post- Gondwana rift evolution)

The second episode of cooling recorded by most samples of the Dom Feliciano
Belt, which started in the Cretaceous (150–70 Ma), was slow and continuous
(Figs. 3–5) without significant sedimentation taking place with less
than 240 m. The rift phase is followed by ascending intrusive bodies through
fractures of the crust. Among them the Serra Geral magmatism, which was
ascended by older structures with NW–SE direction. The Jurassic-Cretaceous
reactivation that culminated in the Serra Geral magmatism, which is related
to the Paraná-Etendeka event occurring 134.5 Ma (Pinto et al., 2011;
Janasi et al., 2011), shows to be the main event that provided enough heat to
erase the fission-tracks of the samples located in the Tijucas Terrane (South
Brazil) and Piedra Alta Terrane (Uruguay) (e.g. samples RS-7 and U-10). In
this case, the AFT ages of 130.2±13.3 Ma (sample RS-7) were interpreted
as the age of spilled magmatism over Southern Brazil and as age of
reactivation of old faults in Uruguay during the rifting extension.

The Central Domain, in the Nico Perez Terrane (sample U-2, Fig. 4), and in
the East Domain, in the Pelotas Batholith (sample PJV-6, Fig. 5), shows
slight reheating episodes at temperatures up to 80 ∘C between 210
and 150 Ma. Thus, the AFT age obtained here is close to this magmatism
episode where these samples recorded the South Atlantic syn-rift phase.

The AFT ages of the sample from Pelotas Batholith (samples PJV-7, PJV-14, and
PJV-11, Fig. 1) possibly denotes the flexural readjustments, cooling and
mechanical accommodation of the oceanic crust of the post-rift phase
(Tomazelli and Villwock, 2000) during the Cretaceous (∼70 Ma).

Finally, during the Eocene (∼40 Ma), a denudation episode was recorded
in most of the studied samples. The calculated denudation rates are between
32 to 66 m Ma−1 in Uruguay and up to 45 m Ma−1 in Southern
Brazil. This restricted interval, in a certain way, may be correlated with
the formation of the Rio Grande Cone. According to Fontana (1989), structures
related to gravitational movements must have affected the region thermally
over the years. During this period, there was a substantial increase of
detrital input in the Brazilian continental margin (Barboza et al., 2008),
with the development of several deltaic progradation systems over time
(Della-Fávera, 2001). Modeling of samples was also shown for the case
that the rock was exhumed above the PAZ during Eocene. If the temperature
increase (i.e. ∼20∘C) in the Paleocene to Eocene was solely
driven by gravity loading, the thickness of the ∼1800 m. In addition,
the AFT of Aiguá Batholith (38±2.5 Ma; sample U-1) indicates normal
re-activation of old Brasiliano/Pan African Shear Zones along of the Paleogene
rifts.

Summarizing, thermal models support the possibility that the basement of the
Dom Feliciano Belt experienced a first cooling during Ordovician to Permian
(500–260 Ma). After, was followed by tectonic activity and
exhumation/denudation during the Triassic to Jurassic (260–150 Ma). During
the Cretaceous occurred a phase of stagnation, renewed sedimentation which
caused subsidence and reheating of parts of the Dom Feliciano Belt. In the
Late Cretaceous to recent gave rise to higher exhumation rates (40 Ma).

The thermochronological study from AFT in samples from Uruguay and Southern
Brazil divided between three domains of Dom Feliciano Belt namely, West,
Central and East can be identified with differentiated exhumation and complex
thermal history of the distinct domains. These three domains are formed at
the Paleoproterozoic rocks and separated by old reactivated fracture zones.
The thermochronological data indicate AFT ages range from 383.4±40.9 to
38±2.5 Ma, which closely matches the range in previous publications. AFT
central ages measured in the West Domain range from 383.4±40.9 to
250±21 Ma. In the Central Domain the AFT central ages range from
196.2±14.7 to 130.2±13.3 Ma, and in the East Domain, the AFT central
ages range from 262.2±29.9 to 38±2.5 Ma. These data allowed to
separate the Western Domain with older ages (> 250 Ma) of the
Eastern Domain, which presents younger ages (< 230 Ma).

Based on thermal history modeling, the Western Domain cooled down very fast
in the Cambrian in the Taquarembó Terrane, Brazil, and during the
Devonian in the Nico Perez Terrane, Uruguay, followed by the Central Domain
in the Camaquã Basin, Brazil. The Eastern Domain began to register
fast-moderate cooling only from the Ordovician in the Pelotas Batholith, at
the same time as occurs reheating in this area. However, in the Western
Domain, a very fast period of reheating occurred in the Piedra Alta Terrane,
Uruguay. During the Carboniferous-Permian a slow period of cooling is
registered in the Western Domain. This phase is assumed to be triggered by
the Gondwanide Orogeny and Gondwana II (Brasiliano Orogeny).

Mesozoic time is characterized in the Dom Feliciano Belt by very complex
temperature evolution with fast and slow cooling and reheating. The samples
of Western Domain show fast cooling in the Triassic, followed by reheating
especially the samples just below the Piedra Alta Terrane. During the
Jurassic, the East Domain undergoes a slow cooling in the Pelotas Batholith,
Brazil, and Cuchilla Dionísio Terrane, Uruguay. The Pelotas Batholith
also show a reheating period that begins in the Jurassic and extends until
the Oligocene. These thermal histories can be related to the formation of
NW-trending grabens and Paraná-Etendeka event. After almost stable
conditions up to Miocene times a final very rapid cooling occurred.

In the Eocene (∼40 Ma), the Dom Feliciano Belt registered a very large
denudation in the Eastern Domain. Applied from the thermal gradients an
overburden of 2640 m can be assumed, suggesting a correlation with the
Rio Grande Cone. In addition, indicate normal re-activation of old
Brasiliano/Pan African Shear Zones along of the Paleogene rifts.

This article is part of the special issue “Earth surveillance
and space-based monitoring of the environment: integrated approaches”. It is
a result of the EGU General Assembly 2018, Vienna, Austria, 8–13
April 2018.

Cristiane H. Gomes acknowledges the National Council for Scientific and
Technological Development – CNPq for the scholarship programme with the
reference 141178/2010-8. The authors acknowledge to CPRM and Votorantin Metal
Company for borehole samples, and Project FAURGS/FINEP/CTPETRO 21.01.0310.00
for basement samples. We are grateful for reviews of Farid Chemale Jr. and
anonymous reviewer that led to improvements in the
manuscript.